1. Field of the Invention
The present invention relates to a semiconductor integrated circuit for an optical sensor.
2. Description of the Related Art
In recent years, in mobile devices such as mobile phones and smartphones, an illuminance sensor and a proximity sensor mounted in a single compact package are installed, for the purpose of adjusting luminance and improving visibility of a display screen in conformance with the environmental light, or for saving power during a telephone call. These sensors are usually covered by a cover member such as a black cover glass, which substantially blocks visible light.
The spectral property of illuminance sensors is required to be as close as possible to the luminosity factor property (a spectral property similar to that of human eyes). To achieve this, there are known two methods.
The first method is to use one photo diode and either a special epoxy for the package, which functions as an optical filter, or a coating that is directly applied onto the photo diode so as to function as a spectral filter. The second method is using two photo diodes. One photo diode is sensitive to visible light and infrared, and the second photo diode is only sensitive to infrared. To obtain the luminosity factor property similar to human eyes, the spectrum property (the measurement value) of the photo diode sensitive to the infrared is subtracted from the spectrum property (the measurement value) of the photo diode sensitive to the visible light and the infrared. Because the calculation is not automatically performed by the sensor, it is necessary to be performed by a CPU or the like. The calculation formula is different depending on the IR contents of each lamp.
In both methods, a slight relative sensitivity remains in the infrared range, to an extent that does not cause a serious influence in normal uses.
However, in many applications (for example mobile phones or TVs), the illuminance sensor is placed behind a cover member such as a panel, or the like, due to design reasons. The panel, or the like, has a very low transmittance in the visible light range and a high transmittance in the infrared range. This affects the overall spectral property of the sensor, because after placing the sensor behind the panel or the like, the sensor may receive much infrared. This causes erroneous measurements similar to a measurement using different light sources. For light sources with less IR content (like a fluorescent light or a LED), only a small amount of visible light can pass the black panel. This is because the visible light is substantially blocked by the black panel.
However, in light sources with high IR content (like an incandescent lamp), the measurement value of the infrared will be very high as long as the illumination is the same. This is because the visible light is substantially blocked by the panel or the like, a great amount of IR will transmit therethrough and cause increased measurement values for the infrared. As a result, adjustments of lighting and backlighting will not be accurate under different light sources.
FIG. 8 illustrates a spectral property of an a photo diode (PD) for an illuminance sensor in a case where the PD is covered with a cover member such as black glass and in comparison with a case where the PD is not covered with the cover member such as the black glass. Then, the sensor becomes relatively sensitive to infrared light after placing it behind the black glass, and an erroneous luminosity measurement is observed.
There is disclosed a technology of performing luminosity factor correction by subtracting the output current of two PDs through a current mirror circuit and performing AD conversion by an AD converter (see, for example, U.S. Laid-open Patent Publication No. 2012/0049048).
Furthermore, there is disclosed a technology of performing AD conversion with different AD converters on the output currents of two photo-sensors having different spectral properties, and performing digital calculation to measure the illuminance (see, for example, Japanese Laid-Open Patent Application No. 2011-58853).